The insect flight muscle (IFM) of Lethocerus has an almost crystalline sarcomere structure and has been the subject of many ultrastructural investigations. Considerably less is known about the biochemical function and regulation of the individual contractile proteins from this muscle. We have purified myosin from IFM of Lethocerus and have studied its ability to translocate actin filaments in an in vitro motility assay. Since some invertebrate myosins are regulated by direct calcium binding to the myosin and others by phosphorylation of the regulatory light chain subunit of myosin, we have tried to study what role the phosphorylation may play in insect flight muscle. Lethocerus myosin from different preparations shows three to four light chains on SDS-PAGE, two to three of which can be phosphorylated either by smooth muscle myosin light chain kinase or by an endogenous myosin light chain kinase. The role of phosphorylation is still unclear at the present time.